The present invention relates to electronically-commutated motors, also known as xe2x80x9cbrushlessxe2x80x9d motors, for applications that require both acceptable efficiency and low cost. A major, although not exclusive, application lies in the automobile industry for driving the feed pumps for servo-motors or actuators with hydraulic liquid under pressure at a flow rate that is highly variable.
Electronically-commutated motors comprise a stator-and-rotor assembly in which one of the elements (generally the stator) has coils generally constituting a three-phase winding and an electronic control module having inputs connected to rotor position sensors (generally Hall sensors) and to a speed reference signal, and including a power circuit for powering the phases from a DC source under the control of a processor.
In general, for reasons of efficiency, the processor is designed to power the phases in a so-called xe2x80x9c120xc2x0xe2x80x9d mode. In this case, the module feeds two phases of a three-phase motor in turns with periodic voltage pulses having a variable duty ratio to regulate speed by pulse width modulation (PWM) and without any overlap between the on periods of the various phases. However, that power supply mode suffers from the drawback of presenting a speed/torque characteristic that is insufficient at high speed, that adapts poorly to operating conditions under certain conditions (e.g. powering a power steering actuator when steering the wheels hard at low speed or while stationary).
A so-called xe2x80x9c180xc2x0xe2x80x9d mode of regulation is also known which gives rise to all three phases being fed with pulses simultaneously. However that mode of control presents poor efficiency when the motor is lightly loaded, particularly because of the presence of a large reactive component which increases losses due to the Joule effect.
An object of the present invention seeks in particular to provide a motor that satisfies practical requirements better than previously known motors, particularly in that it makes it possible simultaneously to obtain good efficiency under low loading and large torque at high speed. To this end, the invention provides an electronically-commutated motor whose processor is designed to feed its phases at a variable duty ratio in order to provide speed regulation, both in a first mode without time overlap between the periods during which the various phases are fed with pulses of electricity, and in a second mode with 33% overlap between feeds to two phases, with changeover from the first mode to the second mode being undertaken when the duty ratio reaches a predetermined value lying in the range 70% to 100%, and with changeover from the second mode to the first mode being performed when the duty ratio drops below another predetermined value. This other value is selected so as to avoid a xe2x80x9chuntingxe2x80x9d phenomenon by creating hysteresis.
To ensure that changeover from the first mode to the second mode takes place smoothly, the duty ratio is changed during this changeover by looking up a new duty ratio in a stored table. Regulation is generally of the proportional integral (PI) type. Changeover from one mode to the other is then advantageously accompanied by modifying the integral term on the basis of a value looked up in a speed correspondence table.
The above characteristics and others will appear better on reading the following description of a particular embodiment given by way of non-limiting example.